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1 DETERMINATION OF CHEMICAL OXYGEN DEMAND (COD) IN WATER AND WASTE WATER. INTRODUCTION The chemical oxygen demand can be considered as an approximate measurement of the theoretical oxygen consumption, i.e., the quantity of oxygen that is consumed by complete oxidation of the total organic compounds into inorganic end products. A great number of organic compounds are oxidized to % by using this method, and for waters where these compounds are predominant, e.g., municipal waste water, the COD-value is a good approximation to the theoretical oxygen consumption. For other waters, which contain large quantities of substances that are difficult to oxidize using this method, the COD-value will be a poor measurement of the theoretical oxygen consumption. This is the case with some industrial effluents. The method is sensitive to some interference, of which the most serious is oxidation of inorganic substances present, such as chloride, nitrite, sulphide and ferrous iron. The interference from chloride can be reduced, but not removed, by adding mercuric sulphate, which will form a soluble complex with chloride. If the proportion of the other interfering ions is known, the result can be corrected. The accuracy of the COD-value will depend on the composition of the water being analyzed. SCOPE The method can be used for waters with a COD-value exceeding 10 mg/l and a chloride concentration lower than 2000 mg/1. Two methods are described for chloride concentrations below 100 and for chloride concentrations below 2000 mg/1. The latter is also used when COD<CI. Both methods include elimination of interferences from 6 mg/l nitrite-nitrogen. PRINCIPLE The sample is boiled under reflux in about 50% sulphuric acid containing a silver catalyst and a known quantity of potassium dichromate. A part of the dichromate is reduced by oxidizable material present in the sample, and the balance is titrated against ammonium iron sulphate. The COD-value is calculated from the consumed quantity of dichromate. EQUIPMENT 100 ml Kjeldahl digestion tubes when using TK 312/320*. 250 ml Kjeldahl digestion tubes when using TK 506/512/520*. Tube stands (2). Air condensers, 1 m long with ground joints/ or water-cooled reflux head. Digestion System TK 506, 512, 520, 312 or 320. Dispensing Units for 5 and 15 ml volumes. Burette/titrating unit.

3 C Digestion acid 10 g Ag 2 SO 4 is dissolved in 1 litre Conc. Sulphuric acid. Complete dissolution may take 1-2 days. D Ferroin Indicator solution. Dissolve g FeSO 4.7H 2 O in distilled water, add g 1.10-Phenanthroline Monohydrate. Shake until dissolved and dilute to 100 ml with distilled water. E Potassium hydrogen phthalate. Potassium hydrogen phthalate is used as a standard substance. Dry the chemical for 1 hour at 105 C. Accurately weigh out g, dissolve in distilled water and dilute to 1000 ml. The theoretical oxygen consumption for this solution is 500 mg/1. The recovery will be around % of the theoretical value. F Sample preservation solution 4M H 2 SO 4. To about 600 ml distilled water, add carefully and with swirling 220 ml Conc. Sulphuric acid. Cool to room temperature and dilute to 1000 ml with distilled water. SAMPLE PREPARATION Analyze sample as soon as possible. If the sample cannot be analyzed immediately, add 10 ml 4M H 2 SO 4 per litre sample (ph<2), and store at 4 C for not longer than one week. Use glass bottles, plastic only if it does not contribute organic material to the sample. Shake the bottle so that any suspended material is dispersed throughout the entire volume before any sample is taken out. DETERMINATION This description is based on the use of air condensers. If water-cooled are used appropriate modification should be carried out. condensers The digestion unit should have a working temperature of 210 C before the reagents are added to the sample. The sample should boil within 5-10 minutes after placing the tube in the block. If it does not, increase the working temperature. The use of boiling rods in the tubes is essential; otherwise uneven boiling may cause splashing of the corrosive reagents. When using air condensers a small fan can be used to facilitate the circulation of cooling air. Samples with COD-values larger than 700 mg/l should be diluted prior to analysis. Such a large COD-value will turn the sample green upon the addition of the digestion acid.

4 If the equipment has not been in use for some time, all the digestion tubes and condensers should be run through a blank digestion before analyzing samples. Samples containing less than 100 mg/l chloride Place the tube stand on the preheated digestion unit. Pipette or dispense 10.0 ml sample into the digestion tube. Make 3 blanks on 10.0 ml distilled water. In each tube, dispense or pipette 5.0 ml M/l dichromate solution and mix. Carefully dispense or pipette 15.0 ml digestion acid into each tube while swirling, and then mix thoroughly. Insert a boiling rod in the tube, and then put on the condenser. Put the tube in the tube stand on the digestion unit, and then continue with the next sample. Digest samples and blanks for 2 hours. Prepare a cooling bath, large enough for a tube stand. After the digestion time is completed, treat the samples as described below, and in the same order as they were put in the digestion unit. Carefully flush a small amount of distilled water (20-25 ml) through the condenser and lift the tube out of the digestion unit and put it in the tube stand in the cooling bath. Rinse the condenser with a further volume (30-35 ml) of distilled water, and then remove it from the tube. Continue with the next tube. When all the tubes are cooled, remove the tube stand from the cooling bath. Transfer the tube content to an Erlenmeyer flask. Add 2-3 drops of Ferroin indicator solution and then titrate the excess dichromate with the M Ammonium Iron Sulphate solution. The colour change will be from blue-green to red-brown. The blue-green colour may reappear within minutes. This is not significant to the analysis. Sample containing mg/l Cl and sample where COD<Cl Weigh out 0.2 g HgSO 4 into each digestion tube, then proceed as described above. Also the blanks shall contain HgSO 4. CALCULATION The Chemical Oxygen Demand is calculated from the formula: COD (mg.o 2 /l) = (a-b)*c*d*1600 V

5 Where a = ml iron (ii) solution used to titrate the blank. b = ml iron (ii) solution used to titrate the sample. c = Molarity of the iron (ii) solution used for titrating. d = ml dichromate ( m) added. e = ml sample or blank. Reduced inorganic substances such as ferrous iron, sulphide, manganese (II) will oxidize stoichimetrically, so if the concentrations of these are known, correction of the COD-value is possible. REMARKS Distilled water should be used throughout. There is a risk that oxygen consuming matter can be released from organic ion-exchangers.

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